Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under a multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as OFDM signal.
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system air interface is organized into subframes of one millisecond (ms) which consists of 12 or 14 OFDM symbols depending on the specific system parameters. As shown in FIG. 5, ten subframes make up one radio frame. First few OFDM symbols of a subframe are used for control channels to manage the allocation of resources in uplink and downlink as shown in FIG. 5. The first few OFDM symbols used for the control channels is referred to as the control region herein. The number of subcarriers in each OFDM symbol varies depending on the specific system parameters. One subcarrier in one OFDM symbol is referred as Resource Element (RE) as shown in FIG. 6. For example, a 3GPP LTE wireless communication system with a 10 MHz channel bandwidth and normal Cyclic Prefix (CP) will have 600 REs and 14 OFDM symbols in one subframe. A single RE may be used to communicate one point from a modulation constellation such as Quadrature Phase Shift Keying (QPSK). In 3GPP LTE wireless communication system, it may be possible to use other modulation constellations for the data region of a subframe, but for the control region only QPSK modulation may be used. Two bits of information may be mapped to a QPSK modulation symbol. Therefore, a single RE can be used to map two bits of information.
A base station in a 3GPP LTE wireless communication system may be referred to as an evolved Node B (eNB). A client terminal in a 3GPP LTE wireless communication system may be referred to as User Equipment (UE). Reference Symbols (RS) are transmitted by an eNB to help a UE to perform channel estimation which may be required for coherent demodulation of the control channels and payload data. The RS are specific REs in a subframe for which the UE knows, a priori, the information bits used for modulating those specific REs. Using the a priori information about the information bits in RS REs, the UE may be able to perform channel estimation to demodulate other REs used for control channel and payload data transmission. One specific arrangement of RS REs is shown in FIG. 7 for the case of a single transmit antenna port at the eNB.
The Physical Downlink Control Channel (PDCCH) is a control channel used by an eNB in 3GPP LTE wireless communication system for the purpose of allocating resources to one or more UEs that may be present in its coverage area. To keep the overhead low for the control messages used for allocating resources, a UE may be need to search a specific portion of the control region of the 3GPP LTE wireless communication system air interface as shown in FIG. 5. The PDCCH is specified in units of Control Channel Elements (CCEs) and the CCEs in turn are specified in units of Resource Element Groups (REGs). Specifically, one REG consists of four REs as shown in FIG. 8 and one CCE consists of nine REGs as shown in FIG. 9. A PDCCH may consist of one, two, four, or eight CCEs a shown in FIG. 10. The number of CCEs used for a PDCCH is referred herein as Aggregation Level (AL), i.e., AL-1, AL-2, AL-4, and AL-8 for one, two, four and eight CCEs. There may be multiple PDCCHs transmitted by an eNB in the control region of a subframe to allocate resources to multiple UEs in a single subframe.
In order to limit the number of PDCCHs the UE may need to attempt to decode to find the specific PDCCH that may contain resource allocation for it, the control region for the PDCCH is organized into Search Spaces. Two types of search spaces are used: Common Search Space (“CSS”) and UE Specific Search Space (“UESSS”). The CSS is a subset of first 16 CCEs from all the available CCEs in the control region of a subframe and the CSS is the same for all UEs served by a given eNB. The UESSS is also a subset of CCEs from all the available CCEs in the control region of a subframe. However, the subset of CCEs used for UESSS is specific to each UE and it may vary from one subframe to the next. The subset of CCEs used in CSS and the subset of CCEs used in UESSS may be disjoint, partially overlapping, or entirely overlapping.
In the CSS, the UE needs to consider only AL-4 and AL-8. There are four different AL-4 candidates and two different AL-8 candidates in CSS. This leads to a total of six PDCCH candidates from AL perspective as illustrated in FIG. 11. In the UESSS, the UE needs to consider all four AL. In a manner similar to CSS, there are six different AL-1 candidates, six different AL-2 candidates, two different AL-4 candidates and two different AL-8 candidates. This leads to a total of 16 UESSS PDCCH candidates as illustrated in FIG. 12.
The payload data describing the resource allocation information that is transmitted using PDCCH is referred to as Downlink Control information (DCI). There are different types of resource allocation for downlink and uplink. The length of the allocation information varies based on the system parameters being used. Therefore, the length of the DCI may vary. The DCI payload in PDCCH is protected by error correction coding (convolutional codes) as well as error detection based on a 16-bit CRC. A given PDCCH candidate may be used to send a DCI of one of two possible different lengths. Since the length of the DCI payload may be different, the FEC encoding and decoding is also different even though the total number of coded bits may be the same for DCI of two different lengths. For example, a DCI of length 27 bits or 33 bits may be used to map to a PDCCH of AL-1.
The PDCCH decoding may be viewed as consisting of two parts, referred herein as RE Processing and Forward Error Correction (FEC) decoding. The RE Processing may include channel estimation, equalization, demodulation and soft channel bits (also known as Log Likelihood Ratios—LLRs) generation for each RE to be processed for a given PDCCH candidate. The FEC decoding may involve error correction of the received soft channel bits and CRC checking.
For a given PDCCH candidate, with example DCI lengths of 27 and 33, the RE Processing may be only performed once. However, the FEC decoding may be performed twice; once for DCI length of 27 and again for DCI length of 33. A similar process may be repeated for all the PDCCH candidates at each AL. This leads to a total of 22 PDCCH candidates between CSS and UESSS and with two FEC decoding per PDCCH candidate which leads to a total of 44 blind decoding attempts.